IsaacGray

It acts as a leader unit for other necromorph hanging back as
Isaac tears through the other necromorphs and assimilateing the
parts he cuts off eventually turning into a big amorphous blob with
slasher blades, lurker quills, puker...puke and so on and so forth.
So it pretty much comes down to having to decide to try and kill a
super agile enemy while his buddies are ripping you a new one or
killing them all quickly and having to deal with a super powerful
boss-like enemy.